MECHANICAL PROTERTIES OF POLYMERS 151 



narrower than in polyisobiityleno, which corrolatcs with the smaller 

 steric hindrance. 



Measurements have also been made in the 70-kc to 140-kc range for 

 two non polar liquids, polybutadiene and polypropylene and one polar 

 liciuid, polypropylene sebacate. The first two have the formulae 



while the polar licjuid, polypropylene sebacate, has the formulae 

 OHHHHHHHH CH3 H 



— O— C— C— C— C— C— C— C— C— C— C— 0— C— C— 



I I I I I I I I II II 

 HHHHHHHHH HH 



The measvu'ed results are given by Table III. These data are plotted 

 on Fig. 22 as a function of the product of frequency times static viscosity. 

 For comparison the data for polyisobutylene polymer F is also plotted. 

 By extrapolating to low values of the product-frequency times static 

 viscosity it is seen that the low frequency "quasi-configurational" stiff- 

 nesses of these liquids run from 10 to 1.5 X 10 dynes per square centi- 

 meter. Polyisobutylene has the greatest stiffness for any value of fre- 

 quency times viscosity while polybutadiene has the least. This reflects 

 the greatest steric hindrance of polyisobutylene and the smallest for 

 polybutadiene which has only hydrogens connected to the carbon chain 

 atoms. Another consequence of the larger steric hindrance of the CH3 

 groups of polyisobutylene is that the viscosity associated with the short- 

 est chain segment motion is largest for polyisobutylene and smallest 

 for polybutadiene as can be seen from the ratio of dynamic to static 

 viscosities for high values of frequency times static viscosity. 



The activation energy for static viscosity are for polypropylene, poly- 

 propylene sebacate and polybutadiene respectively 21.2, 12 and 8 kilo- 

 calories compared to 16 kilocalories per mole for polyisobutylene. The 

 high-frecjuency activation energies as determined by the dynamic meas- 

 urements are respectively 11.8, 4.6 and 1.4 kilocalories for polypropylene, 

 polypropylene sebacate and polybutadiene. The difTerences between the 

 acti\'ation energy for static flow and that for dynamic flow are respec- 

 tively 9.6, 7.4 and 6.6 kilocalories, which values are all higher than 



